[tests] remove duplicate dyn.load, add explanation

tests/tCallNimFromR.R

@@ -1,6 +1,8 @@
-dyn.load("tests/libtNimFromR.so")
-
-source('tNimFromR.R')
+# Source the autogenerated R file, which loads the dynamic
+# library and wraps them as R functions of the same name.
+# IMPORTANT: The path here is based on the location of where the
+# R interpreter is run!
+source('tests/tNimFromR.R')
 
 check <- function(arg) {
     if (!all(arg)){
@@ -31,6 +33,5 @@ check(y == yOrig + 1.0)
 
 printVec(y)
 
-
 checkSexp(x)
-checkSexpRaw(x)
+checkSexpRaw(x)